1. Field of the Invention
This invention relates to a control method and apparatus for a thermal transfer color printer suited for color printing.
2. Description of the Related Arts
In the thermal transfer color printer for printing a color image, it is generally practised that an ink film having different ink layers formed thereon is overlapped on a printing paper and a thermal head is pressed against the ink film and printing paper. The thermal head is formed by the array of a plurality of heating elements, which are heated by a predetermined print signal to activate the ink of the ink film, thereby transferring the ink to the printing paper. This ink film has a role of base film on which for example, yellow, magenta and cyan thermal inks are sequentially coated in respective areas in this order. These inks, when supplied with heat energy from the heating elements, are melted into liquid state or sublimated into gas state and transferred to the printing paper.
For a color print, a color image is decomposed into three color components, and each color ink is transferred by the thermal head to the same area of the printing paper in accordance with the corresponding color component signal of the color image. For example, a yellow image is transferred to the printing paper, and a magenta image is transferred by the thermal head on the yellow-image printed area of the printing paper by using the magenta ink of the ink film. Further, similarly a cyan image is transferred thereto by using the cyan ink of the ink film, completing a multi-color print with gradation.
Therefore, in order to produce a color print of this type with no color shift, it is necessary to precisely carry the ink film and the printing paper.
As one of the methods of carrying the ink film and the printing paper, the ink film is carried in one direction by the rotation of a platen roller, and the printing paper is also carried together with the ink film in one direction (forward direction) during the transfer period but, when one color image has completely been transferred it is carried in the reverse direction back to the original position where the next color image is transferred to the same area of the printing paper. Thus, during transferring, the thermal head is pressed against the surface of the platen roller through the printing paper and the ink film, and the printing paper and the ink film are carried together by the rotation of the platen roller. When the transferring operation is finished, the thermal head is separated from the platen roller, so that only the printing paper can be carried in the reverse direction. One example of the thermal transfer color printer is disclosed in the National Technical Report, "High Speed Thermal Transfer Color Printer" Vol 30, No. 3 June 1984, P. 325 to 332.
Another example of the thermal transfer color printer is disclosed in Japanese Patent Unexamined Publication No. 58-27463 which was filed in Japan by Tokyo Shibaura Denki Inc. on Aug. 11, 1981 and laid open on Feb. 18, 1983. According to this publication, the printer has a platen roller opposite to the print head and a controller for controlling this platen roller, and this controller controls the platen roller to be pressed against the print head or separated therefrom on the basis of a print signal.
For the thermal transfer color printer for color print by superposition of a plurality of color images printed, there is proposed a control system in which a sensor for detecting the position of the printing paper is provided near the platen roller and the detected signal from the sensor is used to control the thermal head to move, the current supply to the head (, or printing operation), the speed at which the printing paper is carried, and so on.